The invention relates to a subscriber station, a network control means and a method for carrying our inter-frequency measurements in a mobile communication system. The invention also relates to a mobile communication system in which such a subscriber station, such a network control means and such a method are employed.
As will be explained with more details below, in a mobile communication system transmission conditions on a connection (communication connection or signaling connection) between a subscriber station and a base transmitter station must be monitored and the need for an inter-frequency or inter-system handover is detected, e.g. when the transmission conditions deteriorate. When the need for an inter-frequency or inter-system handover is detected, an inter-frequency measurement trigger signal is generated to indicate the need for an inter-frequency or inter-system handover and to initiate inter-frequency measurements on a different frequency than currently used. In response to the trigger signal inter-frequency measurements are carried-out by a subscriber station on one or more different frequencies and if a suitable new frequency has been found the actual inter-frequency or inter-system handover or cell-reselection takes place. Hereinafter, the term xe2x80x9chandoverxe2x80x9d is used to designate an inter-frequency handover or an inter-system handover or a cell-reselection, even if not explicitly stated.
When a connection is established between the base transceiver station and the subscriber station, even if only a signaling connection is established with the subscriber station being in an active-mode of operation, there always exists some data traffic on the connection and the subscriber station and the network must carry out inter-frequency measurements when no data is transmitted, since otherwise some of the communicated data on the connection will be lost. Another important aspect is when and how the inter-frequency measurement trigger signal should be generated by the network to initiate the inter-frequency measurements. It should be noted that the inter-frequency measurements themselves are, however, always carried out in the subscriber station in response to the inter-frequency measurement trigger signal.
The present invention in particular addresses the problem which time interval should be used in the subscriber station for carrying-out these inter-frequency measurements.
Hereinafter inter-frequency will be abbreviated with xe2x80x9cIFxe2x80x9d.
With respect to a conventional method for triggering IF-measurements in a mobile communication system, FIG. 1 shows a general overview of a telecommunication system TELE which comprises at least two different mobile communication systems T1, T2. A subscriber station, e.g. a mobile station UE, which is operable in the first mobile communication system T1, may also be operable in the second mobile communication system T2. Within each mobile communication system T1, T2 the mobile station UE can move around different cells S1, S2, S3, S1xe2x80x2, S3xe2x80x2 and C1-C6. Due to different handover criteria the mobile station UE may perform an inter-frequency handover within the same system or an inter-system handover to/from the other system. It should be noted that the present invention is equally well applicable for triggering an inter-frequency handover within the same system and/or an inter-system handover (i.e. intra-frequency, inter-frequency and inter radio-access system measurements) and FIG. 1 only shows two mobile communication systems T1, T2 as an example where both such handover procedures may take place.
FIG. 1 shows as an example for the first mobile communication system T1 a WCDMA (Wideband Code Division Multiple Access) or CDMA (Code Division Multiple Access) communication system comprising a network control means RNC (Radio Network Controller) (which is part of the UTRAN: UMTS terrestrial radio access network), at least one base transceiver station RBS, RBSxe2x80x2 (in WCDMA called radio base station), at least one subscriber station UE (Mobile Station) as well as a number of (possibly) overlapping cells S1, S2, S3, S1xe2x80x2, S3xe2x80x2.
An example for the second mobile communication system T2 is a communication system according to the GSM (Global System for Mobile Communications), PDC (Packet Digital Cellular) and D-AMPS (Digital-Advanced Mobile Personal Service) standards.
In FIG. 1 an example of a GSM system is shown for the second mobile communication system T2. However, it should be noted that the invention can in principle be applied to any type of digital mobile telephone system and is as such not restricted to the aforementioned systems. The GSM system shown in FIG. 1 comprises the conventional units of a base station controller BSC, at least one mobile switching center MSC as well as a gateway mobile switching center GMSC. The mobile stations UE are served by a plurality of base transceiver stations BTS within the cells C1-C6 in which the mobile station UE can move around.
The network control means RNC of the WCDMA system in FIG. 1 is connected via a UMSC unit to the gateway mobile switching center GMSC of the GSM system.
Depending on the geographical layout of the first and second mobile communication systems T1, T2 the cells S1, S2, S3, S1xe2x80x2, S3xe2x80x2 of the first mobile communication system T1 may also completely or partially overlap with the cells C1-C6 of the second mobile communication system T2. Of course, if the mobile station UE is to carry out an inter-system handoverxe2x80x94then the mobile station UE will be able to operate according to the specifications of the first and the second mobile communication system.
One reason for performing inter-frequency or inter-system handovers in the telecommunication system TELE in FIG. 1 may be due to coverage reasons. This is due to the fact that neither the first communication system nor any other system has a complete coverage in all geographical areas, e.g. hot spots in UMTS. Furthermore, some cells within the mobile communication system may operate on frequencies which are not applicable in adjacent cells. Therefore, by letting the mobile station UE (or in fact the network control means RNC) perform either an inter-frequency handover of an inter-system handover, the mobile station UE can be used in a larger area without interruptions in the communication.
Another reason for the handover may be capacity reasons. Either the mobile communication system or other mobile communication systems may become heavily loaded at times, so that an inter-system handover may be required. Analogously, the mobile station UE may have established a connection on a particular frequency and it may be necessary that another frequency is to be used due to decaying transmission conditions. This other frequency may be present within the same cell or in another cell and both are generally termed inter-frequency handover. As indicated in FIG. 1, the inter-frequency measurements (necessary for an inter-frequency handover/or an inter-system handover) is always carried-out by an inter-frequency measurement means IFMM situated in a mobile station UE.
The network control means RNC comprises a paging flag sending means PFSM for sending a paging flag to the mobile station UE when a signaling communication link has already been established between the subscriber station UE and the network. For example, when the mobile station UE has been switched on and has been registered in the network, the subscriber station is in a registered and non-active mode of operation. A standby operation means holds the subscriber station in such a non-active mode of operation. In such a non-active mode of operation the operation of the subscriber station UE is invoked by receiving the paging flag PF from the network control means RNC, namely when a call is pending for the subscriber station UE and when a communication connection is to be set up to the subscriber station UE.
FIG. 2 shows a general flow chart of a method for carrying-out an inter-frequency or inter-system handover in a mobile communication system when a signaling connection or a communication connection is set up. In step ST11 a handover means HORM (HandOveR Means) situated in the network control means RNC or the mobile station UE monitors the network performance regarding the capacity/coverage aspects as explained above. In step ST12 the handover means HORM decides whether in principle a handover is necessary according to the criteria determined in step ST11. If so (xe2x80x9cYxe2x80x9d in step ST12), the mobile station is triggered to perform inter-frequency measurements in step ST13. More particularly, in step ST13 an IF measurement trigger signal IFTS is output by the handover means HORM. The IF-measurements means IFMM can be triggered by a mobile-evaluated-handover trigger signal IFTS or a network-evaluated-handover trigger signal IFTS in step ST13.
In order to perform a fast and reliable inter-frequency handover when there is the need for such a handover, it is advantageous to provide the outputting of a reliable trigger signal IFTS in either the network control means RNC and/or in the mobile station UE. Of course, in order to provide a well-designed trigger procedure, there is not a single triggering condition that needs to be monitored in step ST11 and which will eventually trigger the mobile station UE to perform IF-measurements on other frequencies or systems. Usually, a couple of conditions are monitored in step ST11 and must be fulfilled that the trigger signal is output in step ST13. Such conditions may for example comprise an excessively high output power from either the down-link (network to subscriber station) connection or the up-link (subscriber station to network) connection and/or a high load in the cell. If for example the network detects by measuring the uplink-interference a high load in the cell, it will attempt to trigger IF-measurements and thus a handover to a different cell or a different system. Likewise, if transmission conditions deteriorate, the mobile station UE is triggered to more and more increase its output power and therefore a high output power also indicates the need for IF-measurements and thus the need for a handover.
The prior art reference TS 25 231 V0.3.0, technical specification: Third Generation Partnership Project (3GPP); Technical specification group (TSG), radio access network (RAN); working group 1 (WG 1); Physical Layer-Measurements in the IS 95 standard, dated June 1999 (hereinafter referred to as reference [1]) describes in particular in chapters 3., 4., 5.1.2 a number of conventional measurement trigger criteria. In the mobile communication system described in reference [1] both a network handover means HORM and a subscriber station handover means monitor the performance of the radio-link (RL) and can request a handover. For example, the network handover means HORM monitors the down-link by measurement reports from the subscriber station UE. The network handover means HORM also monitors the traffic load. As explained above, a hand-over evaluated by a mobile station UE is called a mobile-evaluated hand-over, abbreviated MEHO. A hand-over evaluated by the network is called a network-evaluated hand-over, abbreviated NEHO. Since the mobile station UE and the network control means RNC each comprise a handover means each can initiate a handover according to the triggering conditions which are respectively monitored. The four basic criteria during the monitoring in step ST11 in the prior art are the xe2x80x9cbase station traffic load exceededxe2x80x9d condition, the xe2x80x9cdistance limits exceededxe2x80x9d condition, the xe2x80x9cpilot strength below a predetermined thresholdxe2x80x9d condition and the xe2x80x9cpower level exceededxe2x80x9d condition as is described in the aforementioned reference [1].
In response to the generation of an IF measurement trigger signal IFTS in step ST13 (generated by the subscriber handover means HORM or the network handover means HORM), the subscriber station will perform IF measurements in a given time interval in step ST21. As explained above, in order to perform a fast and reliable inter-frequency handover, it is advantageous to let the subscriber station UE perform signal quality measurements on a different frequency, e.g. in a target cell or in a different system, and to report these to the network control means RNC, such that the network control means RNC can base its handover decisions, as to which cell the subscriber station UE is to be handed over, on these reported signal quality measurements. As explained below, the performing of IF-measurements in the subscriber station UE is not a trivial task. For example, in CDMA and FDMA systems the receiver of the subscriber station UE is normally busy receiving information on the current frequency and thus some measurement time has to be created in some way in such systems in order to allow inter-frequency measurements without a drastic loss of data.
Conventional methods for determining a time interval in which field measurements are carried out will be described below with reference to FIGS. 3-1, 3-2, FIGS. 4-1, 4-2.
As shown in FIG. 3-1, in a CDMA communication system the data communication is generally performed by exchanging data frames FR consisting of a plurality of time slots TS1 . . . TS15. Each time slot comprises a control portion CP and a data portion DP. As described in reference [2]: TS 25.201 V2.1.0, a third generation partnership project (3GPP); technical specification group (TSG); radio access network (RAN; working group 1 (WG1); physical layer-general description, dated June 1999, and as also indicated with step ST21xe2x80x2 in FIG. 3-2 and in FIG. 3-1, it also possible to carry out the data transmission in a compressed mode (also called slotted mode) in order to create some time for the IF measurement. For this purpose the network control means RNC comprises a compressed mode setting means in which the data contained in the data portion DP is compressed, i.e. concentrated to a smaller part of the frame, resulting in an idle time portion ITP. The subscriber station UE can comprise a compressed mode determining means which determines i.e. realizesxe2x80x94being informed about the compressed mode of transmission via signaling or some information sent from the compressed mode setting means of the network control means RNCxe2x80x94the compressed mode of operation. If such a compressed mode of operation is detected, the subscriber station UE enters a compressed mode of operation and performs the IF measurements in the idle time IT in step ST21xe2x80x3 in FIG. 3-2.
In a CDMA system such a concentration of information is achieved by reducing the processing gain G=chips/information bits=1/SF, e.g. by decreasing the spreading factor SF. Another possibility how the concentration of information can be achieved is by changing the channel coding scheme, e.g. from r=⅓ to r=xc2xd. Due to the compressed mode of operation a time interval IT is generated in which the IF measurements can be carried out by the IF measurement means IFMM in the subscribed station UE.
FIG. 4-1 and steps SC21xe2x80x2xe2x80x3 and ST21xe2x80x2xe2x80x3xe2x80x2 also show another possibility of how a time interval can be provided in which the field measurements can be carried out. In a GSM system, a specific time slot FMS of a frame consisting of a plurality of TDMA time slots TS1 . . . TS-M is specified and the field measurements are carried-out in the portion FMP. That is, in a GSM system a predetermined field measurement slot is provided in which no data is sent from the network control means or the base station transmitter to the subscriber station UE. This slot has been agreed upon by subscriber station UE and the network control means (i.e. set in the protocol) and is fixed.
As shown in FIG. 2 and in FIGS. 3-1, 4-1, the network control means RNC triggers the mobile station and step ST13 to perform the IF measurements and it will also indicate to the subscriber station UE on which frequency belonging to a different cell or a different system said IF measurements are to be carried-out. The subscriber station SS will report the IF measurements back to the network control means RNC within a predetermined time. Then, in step ST22, the network control means RNC will determine whether a handover to the selected frequency (cell or different system) is possible. If it is not possible, because for example a too high interference is detected on the new frequency, the network control means selects a new target cell (frequency) in step ST23 and the IF measurements are repeated by the subscriber station UE in step ST21. Furthermore, the network control means RNC can order the subscriber station UE to perform a periodic search or a single search. Such a procedure is for example described in reference [1] for a synchronized communication system.
In some systems like CDMA 2000 the subscriber station UE not only reports the IF measurements back to the network control means, but it also indicates to the network control means RNC how long (time-wise) and when (the starting time) the subscriber station UE will be able for performing the desired IF measurements. If the network control means RNC has knowledge of the time-interval in which the subscriber station UE intends to perform the IF measurements, then the network control means RNC can make some provisions to compensate for data frames, which would be sent by the network control means RNC, but which the subscriber station UE would not process in the time interval in which it performs the IF measurements. That is, actually data frames will get lost in the time period in which the subscriber UE performs the field measurements unless further provisions are made.
One possibility to compensate this is that the network control means RNC increases the power before or after the measurement time interval or the intervals. Since the error rate is always evaluated over a plurality of data frames, such an increase of power before and after the measurement time interval enables to keep the overall quality for error rate to an average level which will not exceed the requirements of an average error rate.
Summarizing the above disadvantages of providing a time interval for IF measurements according to the afore described prior art, such provisions of the measurement time interval will result in a decreased quality of service (e.g. due to loss of frames), require a complicated system modification (due to the incorporation of PN sequence generators), and will shorten the lifetime of the battery in the subscriber station UE (if the power is increased before and after the time interval).
European patent application EP 99 117 128.1 describes a subscriber station for performing IF measurements in a predetermined time interval as shown in FIG. 5. That is, a data transmission determining means DTDM in the subscriber station determines a time interval in which no data transmission takes place from the base transceiver station to the subscriber station. This is illustrated with steps ST211, ST212 in FIG. 5. In this patent application the time interval in which no data transmission takes place is determined by the subscriber station by determining in a data frame received from the base transceiver station RBS the power ratio of the information contained in the data portion DP to the information contained in the control portion CP. The subscriber station determines that no data transmission will take place in a time interval corresponding to a predetermined number of data frames or data slots following the output of IF measurement trigger signal, if the determined power ratio is below a predetermined power ratio. A second possibility in this patent application is that the subscriber station monitors the transmission of a paging flag from the base transceiver station RBS. When no transmission of the paging flag is detected, it is determined that a data transmission will not take place in a time interval corresponding to a predetermined number of data frames or data slots after detecting said paging flag.
However, in this method it is still possible that actually data is directed to the subscriber station even if the power ratio is below a predetermined threshold, e.g. due to errors during the data transmission.
In the European patent application EP 99 117 129.9 a subscriber station carries out IF measurements in response to an IF measurement trigger signal sent by the network control means. Furthermore, the network control means RNC comprises an IF measurement time interval indication signal TIIS indicating a time interval of an established connection between the subscriber station and the base transceiver station in which IF measurements are to be carried out by the subscriber station. As indicated with step ST211 in FIG. 6, the network control means determines this time interval as a time interval where a temporary reduction of the quality of service is accepted. For the case of a delay-sensitive data transmission frames in the time interval are deleted and a transmission power on the down-link and the up-link on the communication connection before the beginning of said time interval and/or after the end of said time interval is increased. In case of a loss-sensitive data transmission it is suggested that during the time interval indicated by the network control means the transmission frames are buffered in a transmitter buffer means such that some (empty) frames are available in said time interval for the subscriber station to carry out its IF measurements.
However, in this case a temporary reduction of the quality of a service may still take place, if too many frames need to be buffered or if frames are accidentally are deleted.
The above described disadvantages are again summarized in the block diagram of FIG. 7a. In FIG. 7a the network control means RNC (part of UTRAN: UMTS Terrestrial Radio Access Network) orders the mobile station UE (UE: User Equipment) the on down-link transmission channel DL (FACH) to perform IF measurements and the UE uses these IF measurements to make cell-reselection decisions. As explained above, the measurement control order may contain intra-frequency, inter-frequency and inter-radio access-system measurements (for example, when the mobile communication system SYS is a WCDMA system).
In particular, in a special connected mode, called the cell connected state, cell_FACH-substate, the subscriber station UE is requested to continuously monitor a special channel (the FACH channel) on the down-link DL in order to be able to receive its own down link traffic. As indicated in FIG. 7b, on the monitored FACH channel the network control means plans the insertion of user date UE2, UE1 in frames which are sent from the base transceiver station RBS. This means, that in a heavily loaded cell with an extensive FACH traffic, it is impossible for the subscriber station UE to perform inter-frequency measurements, using one receiver, without losing any FACH information, independent as to whether the special information is intended for the individual subscriber station or for other subscriber stations. The simple reason is that there is no time, since the subscriber station in practice has to read an entire frame before realizing to which subscriber station the data contained in the frame belongs.
As indicated in FIG. 7c, it is also possible that the FACH channel is multiplexed with another channel, the PCH channel, on a so-called secondary common control physical channel SSCPCH. The PCH channel is the paging channel on which paging records are sent from the base transceiver station RBS to the subscriber station. When both channels are multiplexed on the SSCPCH channel, then the PCH channel has priority over the FACH channel (FACH: Forward Access Channel), but on the other hand it is only sent when there is a paging record to be sent to anyone in the cell. In the cell_FACH state the subscriber station UE is paged on the FACH channel and not the PCH channel. Consequently, in principle it is not necessary for the subscriber station UE to read to read the PCH frames.
However, as already explained with reference to FIG. 7b, also in FIG. 7c the subscriber station UE first has to read the entire frame before it can determine whether it is a FACH channel containing possibly data intended for the subscriber station or whether it is a PCH channel which the subscriber station would actually not have to read.
Therefore, the subscriber station UE in a connected mode, cell-connected state, cell_FACH substate, for example in a highly loaded cell, experiences the high traffic on the secondary common control channels all the time. The subscriber station must read and decode every frame. Only when the frame has been decoded the subscriber station knows whether it was a FACH frame or a PCH frame. Only then the subscriber station UE can discard the data of the PCH frame. On the other hand, the FACH frame is read entirely if the subscriber station UE determines that it contains data intended for the subscriber station UE. If the subscriber station UE detects that the FACH frame does not contain any data intended for it, it will afterwards discard the data.
However, if the subscriber station UE would receive a trigger signal to perform IF measurements, it would have to leave the SCCPCH channel to perform the inter-frequencies, i.e. the subscriber station UE cannot read and decode the frames on the SCCPCH channel any longer. This might imply a loss of data because the subscriber station UE will not know what is sent when it performs temporarily its inter-frequency measurements. Thus, the subscriber station can do nothing else but guessing when a data transmission is not directed to it or to use one of the above described methods in FIGS. 1-6 with the above described disadvantages.
Therefore, the object of the present invention is to provide a subscriber station, a network control means, a telecommunication system and a method which allows to perform IF measurements without any loss of data at the subscriber station, not even a temporary degradation of the transmission quality.
This object is solved by a subscriber station of a mobile communication system GSM; WCDMA, SYS including an inter-frequency IF measurement means IFMM adapted to perform IF measurements and a data transmission determining means DTDM adapted to determine a predetermined time interval in which a base transceiver station RBS does not direct to subscriber stations UE1, UE2 a data transmission on at least one down-link transmission channel FACH, PCH, SCCPCH on which data transmissions are performed by sending data frames FR1-FR4 from the base transceiver station RBS to one or more subscriber stations UE1, UE2, wherein said IF measurement means IFMM is adapted to perform said IF measurements in said predetermined time interval, characterized in that said data transmission determining means DTDM includes a measurement opportunity indicator determining means MOPDM for monitoring the receipt of a measurement opportunity indicator MOP transmitted from said base transceiver station RBS and for determining as said predetermined time interval the time interval of at least one frame in said down-link transmission channel FACH, PCH, SCCPCH.
Furthermore, this object is solved by a network control means RNC of a mobile communication system GSM; WCDMA, SYS comprising a plurality of subscriber stations UE each including an inter-frequency IF measurement means IFMM adapted to perform IF measurements and a data transmission determining means DTDM adapted to determine a predetermined time interval in which a base transceiver station RBS of said system does not direct to said subscriber stations UE1, UE2 a data transmission on at least one down-link transmission channel FACH, PCH, SCCPCH on which data transmissions are performed by sending data frames FR1-FR4 from the base transceiver station RBS to one or more subscriber stations UE1, UE2, wherein each said IF measurement means IFMM is adapted to perform said IF measurements in said predetermined time interval.
The object is also solved by a method for performing inter-frequency IF measurements in a subscriber station UE of a mobile communication system GSM; WCDMA, SYS wherein in said subscriber station UE a predetermined time interval is determined in which a base transceiver station RBS will not direct to said subscriber station UE1 a data transmission on at least one down-link transmission channel FACH, PCH, SCCPCH on which data transmissions are performed by sending data frames FR1-FR4 from a base transceiver station RBS to one or more subscriber stations UE1, UE2, characterized by the following steps: monitoring the receipt of a measurement opportunity indicator MOP transmitted from said base transceiver station RBS to said subscriber station UE; and determining as said predetermined time interval the time interval of at least one frame in said down-link transmission channel FACH, PCH, SCCPCH.
Furthermore, the object is solved by a telecommunication system comprising at least one subscriber station and at least one network control means as mentioned above.
According to the invention the network control means contains a network data transmission determining means for determining a time interval of at least one frame in a down-link transmission channel in which a base transceiver station of said system does not direct to said subscriber stations a data transmission. A measurement opportunity indicator sending means contained in the network control means then sends to one or more subscriber stations a measurement opportunity indicator when said network data transmission determining means determines said one or more frames in which no data transmission takes place.
The subscriber station according to the invention contains a data transmission determining means with a measurement opportunity indicator determining means for monitoring the receipt of a measurement opportunity indicator transmitted from said base transceiver station and for determining as said predetermined time interval the time interval of at least frame in said down-link transmission channel beginning with the detection and receipt of said measurement opportunity indicator inserted by the network control means.
Since the network will always indicate to the subscriber station by means of the measurement opportunity indicator when a data transmission to an individual subscriber station or to all subscriber stations is carried out or is not carried out, the loss of any data can be avoided if the IF measurement means in the subscriber station carries out its IF measurements exactly in the time intervals following the receipt and detection of the measurement opportunity indicator.
Preferably, the measurement opportunity indicator is transmitted on the down-link transmission channel at the beginning of a frame or on a separate control channel synchronized to the beginning of a frame in said down-link transmission channel.
It is also possible, that the measurement opportunity indicator inserted at the beginning of a particular frame indicates that only a few frames later no data transmission takes place from the network to the subscriber station.
The down-link transmission channel can include a first type data transmission in which data is sent in frames from the base transceiver station to the subscriber station or a second type of data transmission where paging data is transmitted in frames. Both types of data transmission may be multiplexed on a common transmission channel.
The measurement opportunity indicator can indicate the number of succeeding frames over which the IF measurements should be carried out. Possibly, the measurement opportunity indicator also indicates a subscriber station identification which identifies at least one subscriber station which can carry out IF measurements in said at least one frame following the receipt of the measurement opportunity indicator.
If the mobile communication system is a WCDMA (Wideband Code Division Multiple Access) communication system, the down-link transmission channel is a forward access channel FACH and thus a first type of data transmission is a data frame transmission on said forward access channel. Preferably said second type of data transmission is a page data frame transmission of a page channel PCH.
Said transmission channel on which said first type of data transmission and said second type of data transmission is multiplexed can be a secondary common control physical channel SCCPCH in said WCDMA communication system.
It is also possible to transmit the measurement opportunity indicator on a separate channel, for example a paging indication channel PICH in a WCDMA communication system.
Further advantageous embodiments and improvements of the invention may be taken from the dependent claims. Furthermore, the invention can comprise embodiments resulting from a combination of aspects and features which have been separately described and/or claimed in the description and/or the attached claims.
Hereinafter, embodiments of the invention will be described with reference to the attached drawings.